Guitar Amp Repair - Mods - Jensen Speakers



Cap Job

A "Cap Job" refers to the replacement of electrolytic caps (capacitors) in an amplifier.  To better understand what this means, please read on. 

There are several types of caps used in guitar amplifiers.  Some examples are ceramic disks, foil and plastic, metalized film, foil and wax/oil, electrolytic, silver mica.  Let's take a quick look at each type.

1) Electrolytic - Electrolytic caps contain a paste substance (electrolyte) that dries out over time. They are used as filter caps in the main power supply and bias circuit and bypass caps in the preamp. Normal life can be from a few years to around 40 years in some cases but they are in danger of failing any time over about 15 to 20 years of age.  Our normal cap jobs include the replacement of ALL the electrolytic caps in most cases.  See below for more info. on electrolytic caps.

2) Ceramic disks - Some say that ceramic disks don't sound good. I don't believe they inherently have a poor sound but they are prone to drift out of tolerance with age and temperature. The don't generally die when they get old but replacing them can sometimes improve the sound of an amp if the disks have drifted too much. They can also become microphonic and cause ringing in some amps. Replacing them is not part of a normal "cap job".

3) Foil and plastic - Typically used as coupling caps in amps beginning in the early to mid 60s. Examples are "mustard caps" found in early Marshalls, blue molded caps from blackface Fenders, orange drops and blue drops. They tend to be very stable and rarely need to be replaced.

4) Metalized film - A more modern coupling cap. A thin metallic layer is applied directly to plastic insulation layers in the capacitor. This type is also extremely stable and reliable.

5) Foil and wax paper or oil and paper - These were used extensively up until the early to mid 60s including  the yellow Astron caps in tweed Fenders. They break down and cause many problems including bad tone, damaged tubes and noise. Although not part of a normal cap job, they should always be replaced in old amps. 

6) Silver Mica - Commonly used to replace ceramic disks. They tend to be more stable over time and temperatures.

More about Electrolytic caps.
Over time (15 to 25 years in many cases), electrolytic filter and bypass capacitors (caps) begin to go bad. This causes various problems in your amp including poor sound quality, weak output, loss or reverb, intermittent tremolo, hum or “ghost” notes. Replacing these caps often makes a big difference in the overall performance of your amp (if they are starting to fail). In addition, bad filter caps sometimes leak current and can destroy your expensive power transformer. 
See photos of bad caps below. Electrolytic caps contain a semi-liquid paste that tends to dry out and crystallize over the years. When that happens, the capacitor no longer functions properly. This type of capacitor is used in the main power supply, bias supply and in the cathode portion of the audio signal path. When any of the electrolytic caps fail, there is a strong possibility that the others will soon follow since they are the same age and have been in the same environment. When one of them is being replaced, it's an excellent time to have to others done to prevent the amp from having to go back into the shop soon for each successive cap failure. We recommend having them replaced preventively when the amp reaches about  25 to 30 years of age even if they haven't yet failed. Contrary to popular belief, replacing them in not negatively inpact the tone of your vintage amp. I see a lot of bad electrolytic caps in amps built before 1990. It fairly rare to see them bad in amps built after 1990 except for a few bad batches of caps by Ruby and Illinois Capacitor and a couple of others. I've also seen a few Sprague Atoms go bad after less than 10 years. I've never seen a bad cap made by F & T, Xicon or Nichicon unless it was damaged by the failure of another component. 


Consignment Items (Amps and More)




The Sprague Atom caps were the industry standard for many years but prices for "Atoms" have been going way up.  As of this time, a Cap Job using "Atoms" for most medium to large vintage Fender amps will cost considerabley more that other higher quality options so we don't use them unless you specifically requests them. 

We're finding that other brands of caps often exceed the quality of "Atoms" I have switched  over to some of these such as the F & T brand from Germany, Xicon high-temp caps and others.  This means we can offer higher quality parts at lower prices. 

These caps were removed from a 1962 Blond Tremolux and are typical of what happens to this type of cap after 40 years.  The white powdery substance oozing from the caps is the electrolyte that has begun to crystallize.   Also notice the oily stains on the cardboard indicating leaking oil.  When the electrolyte crystallizes, it expands.  Most electrolytic caps have vent holes to allow the escape of the expanding chemical to keep the cap from exploding when this happens.   That's what is happening to these caps.  Using an amp with caps in this condition is asking for trouble!

Here is a close up of 2 of the caps.  Notice the small dimple starting to form on the left cap.